1. Field of the Invention
The present invention relates generally to a switching power amplifier and particularly to a switching power amplifier for use in driving a speaker to reproduce sounds.
2. Description of the Related Art
Heretofore, there has been proposed a switching power amplifier for driving a speaker to emanate sounds by a pulse width modulated (PWM) signal which are pulse-width-modulated by audio signals.
FIG. 1 of the accompanying drawings shows an output circuit for such a switching power amplifier according to the related art.
As shown in FIG. 1, the output circuit for such a switching power amplifier includes switching signal input terminals 1a, 1b, 1c and 1d to which there are supplied switching signals of pulse width modulated signals (digital signals) shown in FIGS. 2A to 2D, which are pulse-width-modulated by audio signals, respectively. The above switching signal input terminals 1a, 1b, 1c and 1d are connected to the gates of field-effect transistors 2a, 2b, 2c and 2d comprising switching elements, respectively.
The field-effect transistors 2a, 2b, 2c, 2d are turned on when the pulse width modulated signals (PWM signals) shown in FIGS. 2A to 2D are held at high xe2x80x9c1xe2x80x9d level and turned off when the pulse width modulated signals are held at low xe2x80x9c0xe2x80x9d level, respectively,
A power source terminal +Vbb, at which a positive DC voltage is supplied, is connected to the drain of the field-effect transistor 2a, the source of the field-effect transistor 2a is connected to the drain of the field-effect transistor 2b, and the source of the field-effect transistor 2b is connected to the ground.
Another power source terminal +Vbb, at which a positive DC voltage is supplied, is connected to the drain of the field-effect transistor 2c, the source of the field-effect transistor 2c is connected to the drain of the field-effect transistor 2d, and the source of the field-effect transistor 2d is connected to the ground.
A junction between the source of the field-effect transistor 2a and the drain of the field-effect transistor 2b is connected to one end of a speaker 6 through a series circuit of a low-pass filter 3 for converting a PWM signal to provide an analog signal, a resistor 4 for detecting a current and a protecting connection switch 5.
A junction between the source of the field-effect transistor 2c and the drain of the field-effect transistor 2d is connected to the other end of the speaker 6 through a series circuit of a low-pass filter 7 for converting a PWM signal to provide an analog signal, a resistor 8 for detecting a current and a protecting connection switch 9.
A voltage developed across the current detection resistor 4 is supplied to an operational amplifying circuit 10 so that, when a current flowing through the current detection resistor 4 is abnormally large in magnitude, the protecting, e.g. relay connection switch 5 is turned off and that the voltage developed across the current detection resistor 8 is supplied to an operational amplifying circuit 11. When a current flowing through this current detection resistor 8 is abnormally large in magnitude, the protecting, e.g. relay connection switch 9 is turned off.
To a control signal input terminal 12 is supplied a control signal having a predetermined time period when the switching power amplifier is energized or de-energized. When the switching power amplifier is energized or de-energized, the connection switches 5 and 9 are turned off during a predetermined time period so that the speaker 6 can be prevented from generating noise when the switching power amplifier is energized or de-energized.
In the output circuit for the switching power amplifier shown in FIG. 1, when the field-effect transistors 2a and 2d are turned on and the field-effect transistors 2b and 2c are turned off, a current flows from the positive power source terminal +Vbb through the field-effect transistor 2a, the low-pass filter 3, the resistor 4, the connection switch 5, the speaker 6, the connection switch 9, the resistor 8, the low-pass filter 7, the field-effect transistor 2d and the ground, in that order. When the field-effect transistors 2b and 2c are turned on and the field-effect transistors 2a and 2d are turned off, a current flows from the positive power source terminal +Vbb through the field-effect transistor 2c, the low-pass filter 7, the resistor 8, the connection switch 9, the speaker 6, the connection switch 5, the resistor 4, the low-pass filter 3, the field-effect transistor 2b and the ground, in that order, to drive the speaker 6 and emanate sounds from the speaker 6.
In the example of the related-art output circuit for the switching power amplifier shown in FIG. 1, the connection switches 5 and 9 are used to protect the switching power amplifier from the excess current and prevent the speaker 6 from generating noise when the power switching amplifier is energized or de-energized, so that, when the relays, for example, are used as the connection switches 5 and 9, the contacts are interposed in the signal path with the result that quality of sound from the speaker 6 is deteriorated and that operation sounds are generated. Although connection switches having very small contact resistance are selected as the connection switches 5 and 9 in order to drive a low impedance load such as a speaker, such connection switches having very small contact resistance are large in size so that the whole of the power switching amplifier cannot be made compact in size as it is desired. Furthermore, when the connection switches 5 and 9 are formed of relays, it is inevitable that an operation speed of the switching power amplifier is lowered.
In view of the aforesaid aspect, it is an object of the present invention to provide a switching power amplifier in which switching elements can be protected from excess current without using connection switches and in which noise can be prevented from being generated when the power switching amplifier is energized or de-energized.
According to an aspect of the present invention, there is provided a switching power amplifier including first and second switching elements connected in series, in which a load is connected to a junction between the first and second switching elements and in which the first and second switching elements are complementarily turned on and off to thereby drive the load. This switching power amplifier is comprised of control means for controlling the first and second switching elements such that the first and second switching elements are all turned off.
According to the present invention, since the first and second switching elements are all turned off, the output terminal is held at high impedance at that time, which becomes equivalent to the case in which the load is disconnected from the output side. Therefore, the switching power amplifier can be protected from the excess current without using the connection switch.
In accordance with another aspect of the present invention, there is provided a switching power amplifier including first and second switching elements connected in series, third and fourth switching elements connected in series, in which a load is connected between a junction of the first and second switching elements and a junction of the third and fourth switching elements and in which the first, second, third and fourth switching elements are turned on and off to thereby drive the load. This switching power amplifier is comprised of control means for controlling the first, second, third and fourth switching elements such that they are all turned off.
According to the present invention, since the first, second, third and fourth switching elements are all turned off, the output terminal is held at high impedance at that time, which becomes equivalent to the case in which the load is disconnected from the output side. Therefore, the switching power amplifier can be prevented from generating noise without using the connection switch when the switching power amplifier is energized or de-energized